Protein C is a natural anticoagulant glycoprotein which prevents intravascular clot formation. Protein C functions as an anticoagulant when converted to an active serine protease (activated protein C). Activated protein C is formed at the site of the endothelial injury in response to blood clotting and helps limit the size of blood clots. We tested the hypothesis that by temporarily blocking the activation of intrinsic protein C, we could reduce subsequent surgical blood loss from a microvascular surgical wound. The formation of activated protein C was blocked systemically by intravenous administration of a monoclonal antibody (HPC4) which binds to circulating protein C and prevents its conversion to activated protein C. Domestic pigs were blindly pretreated with intravenous HPC4 or saline then underwent partial-thickness skin graft harvesting to create a reproducible microvascular wound. Blood loss was measured from each wound and the hemostatic effect of protein C blockade was compared to intravenous saline alone as well as to topical thrombin or thromboplastin. We found that blocking the activation of protein C significantly (P = 0.005) reduces surgical blood loss in this model by 27% compared to saline control animals. Intravenous HPC4 performed equally as well as topical thrombin or tissue thromboplastin. In addition, topical thrombin acted synergistically with HPC4 to reduce blood loss an additional 44% (P = 0.01) as compared to intravenous HPC4 or topical thromboplastin alone. Autopsies performed 1 week after HPC4 treatment showed no evidence of systemic thrombosis resulting from the protein C blockade.(ABSTRACT TRUNCATED AT 250 WORDS)